1. Field of the Invention
The present invention relates generally to damping devices of the nature used to support vibrating bodies, and more specifically to a damping device which is particularly suited to automotive applications and which features an arrangement which improves the attenuation of vibrations having a frequency which tends to produce reverberation noise in the vehicle cabin.
2. Description of the Prior Art
JP-A-60-104828 describes an example of a damping type unit which takes the form of a hollow elastomeric bushing member which is filled with an electrorheogical fluid (or ERF as it will be referred to hereinafter) and which is further provided with an electrically controlled orifice unit which divides the device into a working chamber and an expansion chamber. With this device, when the bushing is compressed, fluid is displaced from the working chamber into the auxiliary one (defined between the orifice unit and a relatively flexible diaphragm) and vice versa. By applying a high voltage across the electrodes of the orifice, the viscosity of the ERF undergoes a remarkable increase and the fluid assumes an almost solid state. Under these conditions the orifice is either effectively blocked (induced to undergo so called stick) or the restrictive properties thereof remarkably increased.
When this type of arrangement is used to support automotive engines, for example, it is possible to improve the effective vibration damping characteristics of the system defined by the engine, mounts and vehicle chassis, particularly at low frequencies. For example, when the engine is idling or undergoes what shall be referred to as engine shake, the vibrations produced exhibit low frequency, large amplitude characteristics.
However, this type of arrangement has suffered from the drawback that even though the above vibration attenuation tends to be very effective in the vibration frequency range in which idle and shake and vibration occur, the device has proven somewhat ineffective in that the spring constant of the device tends to be raised excessively in the vibration frequency range in which reverberation noise is produced in the vehicle cabin.
In this specification it will be assumed for the sake of discussion that the engine "shake" vibrations exhibit a frequency in the order of 10 Hz while the engine "idling" vibrations exhibit a frequency in the order of 25 Hz. On the other hand, the vibrational range in which the reverberation tends to occur lies in the 80-100 Hz region.
More specifically, with the above type of engine mount, in order to attenuate low frequency vibrations which tend to be transmitted between the engine and the vehicle chassis during engine idling and engine shake modes, it is necessary that the dynamic spring constant be set to a low value.
On the other hand, in the vibration frequency range wherein road vibration induces the power unit to resonate, the large amplitude vibration which the engine undergoes (engine shake) makes it necessary to provide the device with a large loss factor in order to control the relatively large displacement of the power unit.
In this connection the above mentioned prior art engine mount arrangement has made use of the "electrode orifice" arrangement which enables the viscosity of the ERF in the orifice passage structure to be controlled in a manner which achieves both of the above mentioned requirements.
However, as the device has only one auxiliary chamber and one orifice providing communication between the main and auxiliary chambers, it is necessary to tune the arrangement for low frequency vibration attenuation and it is therefore difficult to achieve a low dynamic spring constant in relatively high frequency zones. Accordingly, the reduction of the vibration transmission ratio under such conditions is not possible.
Under these conditions, the resonance characteristics of the brackets which support the power unit and drive train are such that, with the increase in the dynamic spring constant, high frequency vibrations tend to be transmitted to the vehicle cabin and induce reverberation noise therein.